Preparation of Highly Transparent (at 450–800 nm) SnO 2 Homojunction by Solution Method and Its Photoresponse

: High-quality SnO 2 :Si ﬁlms and SnO 2 :10 at.% Ga ﬁlms were prepared by the solution method. The roughness of ﬁlms is below 1.08 nm, and possess exceptional transparency ( > 75%) and decent semiconductor properties. Based on this, the SnO 2 :Si / SnO 2 : Ga homojunctions with di ﬀ erent Si doping concentrations were prepared. It is found that the conductivity of the SnO 2 :Si thin ﬁlm gradually increases, and the rectiﬁcation characteristics of the homojunction are optimized with increasing Si doping content. The SnO 2 :15 at.% Si / SnO 2 :10 at.% Ga homogeneous junction has the best performance, the turn-on voltage is as low as 5.6 V, and it also exhibits good unidirectional conductivity. The photoresponse of the SnO 2 :15 at.% Si / SnO 2 :10 at.% Ga homojunction under the lights of red, yellow, and purple was explored respectively. The result shows that the device responds strongly to purple light. Compared with the test results in the dark environment, the device current increases by two orders, which is expected to be applied in the ﬁeld of near-ultraviolet detection.


Introduction
In recent years, with the rapid development of optoelectronic technology, transparent optoelectronic devices have been widely used in many fields due to their excellent optoelectronic properties, such as solar cells [1], light emitting diodes [2,3], photodetectors [4,5], and so on. The highly transparent PN junction is the basic component of many transparent optoelectronic devices. Therefore, the development of low-cost, high-performance transparent PN junctions is the key to the popularization of transparent optoelectronic devices. Transparent semiconductor oxide (TSO) has both excellent transparency and conductivity, which is an excellent material for preparing transparent PN junctions. The current mainstream TSO materials mainly include tin oxide (SnO 2 ), zinc oxide (ZnO), indium oxide (In 2 O 3 ), etc. However, indium is a precious metal, which is expensive and toxic, limiting its market size; ZnO has poor stability and is not resistant to acid and alkali corrosion, which limits its application; SnO 2 is non-toxic, environmentally friendly, low-cost, stable and its mechanical wear resistance is good. At the same time, the optical band gap is 3.67 eV and the 1transparency is excellent. Based on these, SnO 2 has attracted the attention of scholars. Taking advantage of the excellent UV solution with the atomic ratio of Si/Sn of 5%, 10%, and 15%, respectively. Similarly, Gallium (III) Nitrate Hydrate (Ga(NO 3 ) 3 ·xH 2 O, Macklin, analysis pure 98%) and SnO 2 precursor solution were used to prepare a 10 at.% Ga-doped SnO 2 solution.
Glass substrates (1 cm × 1 cm) were treated with a plasma with a power of 120 W for 10 min, then 50 mL of SnO 2 :Si precursor solutions with different doping concentrations were added dropwise to the glass substrate respectively, and then spun coating by homogenizer (model: KW-4A) at 5000 rpm for 12 s. This process was repeated three times to obtain a wet film, and then the film was annealed at 100 • C for 10 min for curing, and then annealed at 300 • C for 1 h to obtain SnO 2 :Si films with different Si concentrations. The same process was used to prepare SnO 2 :10 at.% Ga thin films. The above process was also used to sequentially prepare the SnO 2 :10 at.% Ga film and SnO 2 :Si film with different doping concentrations on the ITO substrate (1 cm × 1 cm) in turn, and the 80 nm aluminum electrode was prepared on SnO 2 :Si film by the thermal evaporation method, then, the tin oxide homojunction was successfully prepared. The structure diagram of the homojunction is shown in Figure 1.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 12 Hydrate (Ga(NO3)3·xH2O, Macklin, analysis pure 98%) and SnO2 precursor solution were used to prepare a 10 at.% Ga-doped SnO2 solution. Glass substrates (1 cm × 1 cm) were treated with a plasma with a power of 120 W for 10 min, then 50 mL of SnO2:Si precursor solutions with different doping concentrations were added dropwise to the glass substrate respectively, and then spun coating by homogenizer (model: KW-4A) at 5000 rpm for 12 s. This process was repeated three times to obtain a wet film, and then the film was annealed at 100 °C for 10 min for curing, and then annealed at 300 °C for 1 h to obtain SnO2:Si films with different Si concentrations. The same process was used to prepare SnO2:10 at.% Ga thin films. The above process was also used to sequentially prepare the SnO2:10 at.% Ga film and SnO2:Si film with different doping concentrations on the ITO substrate (1 cm × 1 cm) in turn, and the 80 nm aluminum electrode was prepared on SnO2:Si film by the thermal evaporation method, then, the tin oxide homojunction was successfully prepared. The structure diagram of the homojunction is shown in Figure 1. The morphology of SnO2:10 at.% Ga thin films and SnO2:Si thin films prepared on glass substrates was characterized using atomic force microscope (AFM) (BY3000, Being Nano-Instruments Ltd., Guangzhong, China). The transmittance of SnO2:10 at.% Ga thin films and SnO2: Si thin films prepared on glass substrates was measured with ultraviolet-visible spectrophotometer (Shimadzu UV-2600, Shimadzu Corporation, Shimadzu, Japan). Time-resolved microwave photoconductive decay (μ-PCD) (LTA-1620SP, KOBELCO, Kobe, Japan) technology was used to test the semiconductor properties of SnO2:10 at.% Ga thin films and SnO2:Si thin films prepared on glass substrates. The electrical performance of all thin film samples and the homojunction of tin oxide, as well as the current response of the homojunction under different frequency light excitation were studied through the semiconductor parameter analyzer (Agilent 4155C, Dongguan nuozhan electronic instrument co. Ltd., Dongguan, China). Figure 2 shows the AFM images of the SnO2:Si and SnO2:10 at.% Ga thin films at 5 μm × 5 μm scale. As can be seen in those images, there are no obvious physical defects such as cracks and holes on the surface of all films. All films are smooth and flat, which can effectively ensure the normal operation of the homojunction. The root mean square roughness (Sq) of the SnO2:10 at.% Ga film is as low as 0.93 nm, which provides good conditions for the growth of the upper SnO2:Si film. The roughness of SnO2:Si films is lower than that of pure SnO2 film, implying that a proper amount of Si doping is beneficial to improve the surface morphology of SnO2 film; the particles and bumps on the surface of the film grow with increasing Si doping concentrations, which finally causes greater roughness of the SnO2:Si film. The morphology of SnO 2 :10 at.% Ga thin films and SnO 2 :Si thin films prepared on glass substrates was characterized using atomic force microscope (AFM) (BY3000, Being Nano-Instruments Ltd., Guangzhong, China). The transmittance of SnO 2 :10 at.% Ga thin films and SnO 2 : Si thin films prepared on glass substrates was measured with ultraviolet-visible spectrophotometer (Shimadzu UV-2600, Shimadzu Corporation, Shimadzu, Japan). Time-resolved microwave photoconductive decay (µ-PCD) (LTA-1620SP, KOBELCO, Kobe, Japan) technology was used to test the semiconductor properties of SnO 2 :10 at.% Ga thin films and SnO 2 :Si thin films prepared on glass substrates. The electrical performance of all thin film samples and the homojunction of tin oxide, as well as the current response of the homojunction under different frequency light excitation were studied through the semiconductor parameter analyzer (Agilent 4155C, Dongguan nuozhan electronic instrument co. Ltd., Dongguan, China). Figure 2 shows the AFM images of the SnO 2 :Si and SnO 2 :10 at.% Ga thin films at 5 µm × 5 µm scale. As can be seen in those images, there are no obvious physical defects such as cracks and holes on the surface of all films. All films are smooth and flat, which can effectively ensure the normal operation of the homojunction. The root mean square roughness (Sq) of the SnO 2 :10 at.% Ga film is as low as 0.93 nm, which provides good conditions for the growth of the upper SnO 2 :Si film. The roughness of SnO 2 :Si films is lower than that of pure SnO 2 film, implying that a proper amount of Si doping is beneficial to improve the surface morphology of SnO 2 film; the particles and bumps on the surface of the film grow with increasing Si doping concentrations, which finally causes greater roughness of the SnO 2 :Si film.

Optical Characterization
The optical transmittance spectra (300-800 nm) of four concentrations of Si in the SnO2:Si films are illustrated in Figure 3. All films have excellent transparency and the transmittance in the visible region exceeds 75%. The transmittance values of the films at the wavelength of 560 nm are shown in Table 1. When the Si doping concentration is 5 at.%, the transmittance of the thin film is the best, exceeding 78%, which indicates that a proper amount of Si doping will help improve the transparency of the SnO2 film. Then, as the doping concentrations continue to increase, the transmittance decreases. However, in the near-ultraviolet band (300-450 nm), the transmittance of the SnO2:Si film is significantly reduced, indicating that there is a strong absorption effect of light in this band. As the spectral wavelength decreases, the transmittance of SnO2:Si films show a gradually decreasing trend, which is attributed to the increased absorption of light in near-ultraviolet band.

Optical Characterization
The optical transmittance spectra (300-800 nm) of four concentrations of Si in the SnO 2 :Si films are illustrated in Figure 3. All films have excellent transparency and the transmittance in the visible region exceeds 75%. The transmittance values of the films at the wavelength of 560 nm are shown in Table 1. When the Si doping concentration is 5 at.%, the transmittance of the thin film is the best, exceeding 78%, which indicates that a proper amount of Si doping will help improve the transparency of the SnO 2 film. Then, as the doping concentrations continue to increase, the transmittance decreases. However, in the near-ultraviolet band (300-450 nm), the transmittance of the SnO 2 :Si film is significantly reduced, indicating that there is a strong absorption effect of light in this band. As the spectral wavelength decreases, the transmittance of SnO 2 :Si films show a gradually decreasing trend, which is attributed to the increased absorption of light in near-ultraviolet band.

Optical Characterization
The optical transmittance spectra (300-800 nm) of four concentrations of Si in the SnO2:Si films are illustrated in Figure 3. All films have excellent transparency and the transmittance in the visible region exceeds 75%. The transmittance values of the films at the wavelength of 560 nm are shown in Table 1. When the Si doping concentration is 5 at.%, the transmittance of the thin film is the best, exceeding 78%, which indicates that a proper amount of Si doping will help improve the transparency of the SnO2 film. Then, as the doping concentrations continue to increase, the transmittance decreases. However, in the near-ultraviolet band (300-450 nm), the transmittance of the SnO2:Si film is significantly reduced, indicating that there is a strong absorption effect of light in this band. As the spectral wavelength decreases, the transmittance of SnO2:Si films show a gradually decreasing trend, which is attributed to the increased absorption of light in near-ultraviolet band.   The optical band gap (E g ) values are calculated from the extrapolation of linear line portion of the plot of (αhυ) 2 versus (hυ), as shown in the Figure 4 for the prepared films. The absorption coefficient (α) and incident photon energy (hν) are linked by the following equation [18]: where A is the constant and E g is the optical band gap. The calculated band gaps of 0 at.%, 5 at.%, 10 at.% and 15 at.% Si-doped SnO 2 films are 3.60, 3.61, 3.56, and 3.58 eV respectively. The optical band gap width of 5 at.% Si-doped SnO 2 is larger than that of pure SnO 2 , which may be caused by the Burstein-Moss effect [19]. With an increasing Si doping level, the carrier concentration of SnO 2 :Si film increases, and the rise in the Fermi level in the semiconductor causes the band gap to increase. However, when the doping concentrations continue to increase, the band gap becomes narrower, which may be due to the multibody interaction between free carriers or between free carriers and ionized impurities [20,21]. At the same time, the optical transmittance spectrum of the SnO 2 :10 at.% Ga thin film was measured, and the optical band gap of the SnO 2 :10 at.% Ga thin film is 3.54 eV, calculated by Equation (1), as shown in Figure 5. It is observed that the SnO 2 :10 at.% Ga film also has high transparency in the visible light range, and the transmittance exceeds 71.4%.
Coatings 2020, 10, x FOR PEER REVIEW 5 of 12 The optical band gap ( ) values are calculated from the extrapolation of linear line portion of the plot of (αℎυ) 2 versus (ℎ ), as shown in the Figure 4 for the prepared films. The absorption coefficient (α) and incident photon energy (ℎ ) are linked by the following equation [18] ： where A is the constant and is the optical band gap. The calculated band gaps of 0 at.%, 5 at.%, 10 at.% and 15 at.% Si-doped SnO2 films are 3.60, 3.61, 3.56, and 3.58 eV respectively. The optical band gap width of 5 at.% Si-doped SnO2 is larger than that of pure SnO2, which may be caused by the Burstein-Moss effect [19]. With an increasing Si doping level, the carrier concentration of SnO2:Si film increases, and the rise in the Fermi level in the semiconductor causes the band gap to increase. However, when the doping concentrations continue to increase, the band gap becomes narrower, which may be due to the multibody interaction between free carriers or between free carriers and ionized impurities [20,21].  At the same time, the optical transmittance spectrum of the SnO2:10 at.% Ga thin film was measured, and the optical band gap of the SnO2:10 at.% Ga thin film is 3.54 eV, calculated by Equation (1), as shown in Figure 5. It is observed that the SnO2:10 at.% Ga film also has high transparency in the visible light range, and the transmittance exceeds 71.4%.   The optical band gap ( ) values are calculated from the extrapolation of linear line portion of the plot of (αℎυ) 2 versus (ℎ ), as shown in the Figure 4 for the prepared films. The absorption coefficient (α) and incident photon energy (ℎ ) are linked by the following equation [18] ： where A is the constant and is the optical band gap. The calculated band gaps of 0 at.%, 5 at.%, 10 at.% and 15 at.% Si-doped SnO2 films are 3.60, 3.61, 3.56, and 3.58 eV respectively. The optical band gap width of 5 at.% Si-doped SnO2 is larger than that of pure SnO2, which may be caused by the Burstein-Moss effect [19]. With an increasing Si doping level, the carrier concentration of SnO2:Si film increases, and the rise in the Fermi level in the semiconductor causes the band gap to increase. However, when the doping concentrations continue to increase, the band gap becomes narrower, which may be due to the multibody interaction between free carriers or between free carriers and ionized impurities [20,21].  At the same time, the optical transmittance spectrum of the SnO2:10 at.% Ga thin film was measured, and the optical band gap of the SnO2:10 at.% Ga thin film is 3.54 eV, calculated by Equation (1), as shown in Figure 5. It is observed that the SnO2:10 at.% Ga film also has high transparency in the visible light range, and the transmittance exceeds 71.4%.

Characterization of Semiconductor Properties of Thin Films
The semiconductor properties of the films were tested by µ-PCD. The shallow localized states, mid-gap states, and the deep localized states of semiconductor oxide have a great influence on the carrier concentration and mobility [22,23]. Shallow localized states can be evaluated by D values, and a high D value indicates low density of shallow local defects; mid-gap states can be evaluated by mean peak value, and a high peak value indicates low density of mid-gap defects; in addition, the deep localized states can be reflected by the fast decay time (that is, the minority carriers lifetime [24]) in the thin-film photoconductivity test [25][26][27][28].
The photoelectric response curve of the SnO 2 :Si thin films is shown in Figure 6, and the mean peak value, D value and minority carrier lifetime of the thin films are extracted from the light response curve, as shown in Table 2. Compared with the shorter rapid decay time (0.05 µs) [25] reported previously, all SnO 2 :Si films have a longer rapid decay time (≥0.424 µs), indicating the density of deep localized states is at a lower level. Furthermore, with the increase of the Si doping concentration, the mean peak value gradually increases, which means that the increase of the Si doping level is beneficial to suppress the generation of mid-gap states. At the same time, the D value of the SnO 2 :10 at.% Si film and SnO 2 :15 at.% Si film in SnO 2 :Si films is relatively large, which indicates that the density of shallow localized defects is low at this time; for a PN junction, it is beneficial to reduce the carrier surface recombination probability, and make the formation of space charge area more favorable. Taken together, the SnO 2 :15 at.% Si film may be more suitable as the n-type layer of the PN junction.

Characterization of Semiconductor Properties of Thin Films
The semiconductor properties of the films were tested by μ-PCD. The shallow localized states, mid-gap states, and the deep localized states of semiconductor oxide have a great influence on the carrier concentration and mobility [22,23]. Shallow localized states can be evaluated by D values, and a high D value indicates low density of shallow local defects; mid-gap states can be evaluated by mean peak value, and a high peak value indicates low density of mid-gap defects; in addition, the deep localized states can be reflected by the fast decay time (that is, the minority carriers lifetime [24]) in the thin-film photoconductivity test [25][26][27][28].
The photoelectric response curve of the SnO2:Si thin films is shown in Figure 6, and the mean peak value, D value and minority carrier lifetime of the thin films are extracted from the light response curve, as shown in Table 2. Compared with the shorter rapid decay time (0.05 μs) [25] reported previously, all SnO2:Si films have a longer rapid decay time (≥0.424 μs), indicating the density of deep localized states is at a lower level. Furthermore, with the increase of the Si doping concentration, the mean peak value gradually increases, which means that the increase of the Si doping level is beneficial to suppress the generation of mid-gap states. At the same time, the D value of the SnO2:10 at.% Si film and SnO2:15 at.% Si film in SnO2:Si films is relatively large, which indicates that the density of shallow localized defects is low at this time; for a PN junction, it is beneficial to reduce the carrier surface recombination probability, and make the formation of space charge area more favorable. Taken together, the SnO2:15 at.% Si film may be more suitable as the n-type layer of the PN junction.

Characterization of SnO 2 Homojunction with Different Si Doping Concentrations
The electrical characteristics of the SnO 2 :10 at.% Ga and SnO 2 : Si thin films with different Si doping concentrations were tested by the semiconductor parameter analyzer. As shown in Figure 7, the I-V curves of all samples have a good linear relationship. The current magnitude of SnO 2 :10 at.% Ga and pure SnO 2 thin film reaches 10 −5 at 5 V. From the Figure 7a, it is established that Si doping can effectively improve the electrical conductivity of SnO 2 thin film, and current magnitude of the film reaches 10 −4 at 15 at.% Si doping concentration. In the literature of Tsay et al. [10], the SnO 2 :10 at.% Ga thin film subjected to post-annealing at 300 • C is p-type conductive.

Characterization of SnO2 Homojunction with Different Si Doping Concentrations
The electrical characteristics of the SnO2:10 at.% Ga and SnO2: Si thin films with different Si doping concentrations were tested by the semiconductor parameter analyzer. As shown in Figure 7, the I-V curves of all samples have a good linear relationship. The current magnitude of SnO2:10 at.% Ga and pure SnO2 thin film reaches 10 −5 at 5 V. From the Figure 7a, it is established that Si doping can effectively improve the electrical conductivity of SnO2 thin film, and current magnitude of the film reaches 10 −4 at 15 at.% Si doping concentration. In the literature of Tsay et al. [10], the SnO2:10 at.% Ga thin film subjected to post-annealing at 300 °C is p-type conductive.  Figure 8 shows the I-V characters of homojunctions prepared based on SnO2:10 at.% Ga and SnO2:Si characterized by semiconductor parameter analyzer. It can be found that the prepared devices exhibit homojunction electrical properties, proving that the SnO2:Si thin film is n-type conductive.  Figure 8 shows the I-V characters of homojunctions prepared based on SnO 2 :10 at.% Ga and SnO 2 :Si characterized by semiconductor parameter analyzer. It can be found that the prepared devices exhibit homojunction electrical properties, proving that the SnO 2 :Si thin film is n-type conductive.
As shown in Figure 8, as the Si doping concentration increases, the rectification characteristics of the homojunction are significantly optimized. When the Si doping concentration is 15 at.%, the rectification characteristic of the homojunction is the best, showing a good unidirectional conduction characteristic. Moreover, the turn-on voltage of is 8.0, 6.4, 5.6 V respectively for 5 at.%, 10 at.%, 15 at.% Si-doped SnO 2 :Si/SnO 2 :10at.% Ga homojunction, as the Si doping concentration increases, the turn-on voltage of the homojunction decreases. It shows that Si doping is beneficial to improve the working performance of the SnO 2 homojunction, and makes the homojunction exhibit more obvious rectification and unidirectional conduction characteristics.
When a forward bias voltage is applied, the electrons and holes inside the homojunction move to the space charge region, then the space charge region is narrowed, and the electron-hole pair recombination generates a current. In this process, the difference in the conductive properties of the n-type and p-type thin films will have a reverse blocking effect on the current [29]. Among all homojunctions, the SnO 2 /SnO 2 :10 at.% Ga homojunction has the largest response current (on the order of 10 −5 A), which is likely to be related to the same magnitude of conductivity of the SnO 2 and SnO 2 :10at.% Ga thin films. The SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction exhibits the best rectification characteristics, which may be related to the minority carrier lifetime of the SnO 2 : Si thin film. Generally, the longer the minority carrier lifetime, the better the rectification characteristics of the homojunction. In Table 2, when the Si doping concentration is 15 at.%, the minority carrier lifetime of the SnO 2 :Si film is the longest (0.626 µs), which confirms the presumption very well. In addition, as the Si doping concentration increases, the conductivity of the SnO 2 :Si film increases, and the rectification characteristic of the homojunction shows an optimization of synchronization. This may be due to the increase of the difference in conductivity between the SnO 2 :Si film and the SnO 2 :Ga film, which leads to an imbalance in the carrier concentration of both sides, and a large number of carriers are injected into the SnO 2 :Ga film, leading to this phenomenon.  Figure 8 shows the I-V characters of homojunctions prepared based on SnO2:10 at.% Ga and SnO2:Si characterized by semiconductor parameter analyzer. It can be found that the prepared devices exhibit homojunction electrical properties, proving that the SnO2:Si thin film is n-type conductive. As shown in Figure 8, as the Si doping concentration increases, the rectification characteristics of the homojunction are significantly optimized. When the Si doping concentration is 15 at.%, the rectification characteristic of the homojunction is the best, showing a good unidirectional conduction characteristic. Moreover, the turn-on voltage of is 8.0, 6.4, 5.6 V respectively for 5 at.%, 10 at.%, 15 at.% Si-doped SnO2:Si/SnO2:10at.% Ga homojunction, as the Si doping concentration increases, the turn-on voltage of the homojunction decreases. It shows that Si doping is beneficial to improve the working performance of the SnO2 homojunction, and makes the homojunction exhibit more obvious rectification and unidirectional conduction characteristics.
When a forward bias voltage is applied, the electrons and holes inside the homojunction move to the space charge region, then the space charge region is narrowed, and the electron-hole pair recombination generates a current. In this process, the difference in the conductive properties of the n-type and p-type thin films will have a reverse blocking effect on the current [29]. Among all homojunctions, the SnO2/SnO2:10 at.% Ga homojunction has the largest response current (on the order of 10 −5 A), which is likely to be related to the same magnitude of conductivity of the SnO2 and SnO2:10at.% Ga thin films. The SnO2:15 at.% Si/SnO2:10 at.% Ga homojunction exhibits the best rectification characteristics, which may be related to the minority carrier lifetime of the SnO2: Si thin film. Generally, the longer the minority carrier lifetime, the better the rectification characteristics of the homojunction. In Table 2, when the Si doping concentration is 15 at.%, the minority carrier lifetime of the SnO2:Si film is the longest (0.626 μs), which confirms the presumption very well. In addition, as the Si doping concentration increases, the conductivity of the SnO2:Si film increases, and the rectification characteristic of the homojunction shows an optimization of synchronization. This may be due to the increase of the difference in conductivity between the SnO2:Si film and the SnO2:Ga film,

Current Response of SnO 2 Homojunction under Different Frequency Lights
The small localized states density of the film can greatly reduce the possibility of photo-generated carriers and defects recombination, as well as provide a favorable environment for the movement of photo-generated carriers. Combined with Figure 8, since the performance of the SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction is the most excellent, it was selected as the sample for the optical response test. By controlling the center distance and angle between the light and the homojunction, the illuminance is uniformly 1200 ± 100 lx, and the three colors of light are irradiated to the surface of SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction as the excitation source, as shown in Figure 9.  The I-V characteristics of the homojunction under the above condition are plotted in Figure  11.  Figure 10 shows the spectrum of light of different colors used in the experiment. The red light is mainly centered at a wavelength of 630 nm; the band of yellow light is mainly distributed from 550 to 700 nm, with the peak at 650 nm; the purple light is centered at a wavelength of 420 nm.
Coatings 2020, 10, x FOR PEER REVIEW 9 of 12 Figure 9. Schematic diagram of a sample illuminated by a light source. Figure 10 shows the spectrum of light of different colors used in the experiment. The red light is mainly centered at a wavelength of 630 nm; the band of yellow light is mainly distributed from 550 to 700 nm, with the peak at 650 nm; the purple light is centered at a wavelength of 420 nm. The I-V characteristics of the homojunction under the above condition are plotted in Figure  11. The I-V characteristics of the homojunction under the above condition are plotted in Figure 11. In a dark environment, the response current is of the order of 10 −9 , and the response current of the homojunction excited by red, yellow, and purple light is significantly improved by a maximum of two orders of magnitude. With the increase of the frequency of light, the response current of the homojunction gradually increases. This may be due to the increase of the concentration of photogenerated carriers, which increases the probability of carriers moving to the space charge region to a certain extent and reduces the on-resistance of the homojunction. The excitation of the purple light increases the response current of the homojunction by two orders of magnitude, which is due to: (1) the SnO2: Si film has a significantly stronger absorption of purple light than red and yellow light; (2) the energy of purple light is higher than that of red and yellow light, resulting in a significant increase in photocarrier concentration. In addition, the turn-on voltage of the homojunction under the purple light excitation is significantly reduced to 5.0 V, and the homojunction shows more excellent rectification characteristics. The open circuit voltage (VOC) is about 1.5 V and the short-circuit current (ISC) is about 1.23 × 10 −10 A in this analysis. Compared with the UV detector based on p-NiO/n-ZnO structure prepared by Zhang et al. [30], which shows the performance that the photogeneration current increases to dark current about twice with ultraviolet light illumination and the VOC is about 0.43 V, the SnO2:15 at.% Si/SnO2:10 at.% Ga homojunction in this research shows better photodetector performance. Besides, the SnO2:15 at.% Si/SnO2:10 at.% Ga homojunction has weak light response to the red and yellow light bands, which is conductive to enhancing the reliability of the photodetector. The above experimental results show that the SnO2:15 at.% Si/SnO2:10 at.% Ga homojunction has the potential to be used as a near-ultraviolet detector.

Conclusions
In this paper, a high-quality transparent SnO2:Si film was prepared by the solution method, and the effect of Si doping concentrations on the structure and properties of the SnO2 film was studied. It was found that: (1) a proper amount of Si doping can reduce the film roughness to as low as 0.74 nm (SnO2:5 at.% Si); (2) a proper amount of Si doping can improve the transparency of the film, up to 84.6% at 560 nm for SnO2:5 at.% Si film; the absorption of light in the near-ultraviolet region enhances for SnO2:Si film with increasing Si doping concentrations; (3) the μ-PCD test shows that the mean peak and D value of SnO2:15 at.% Si is high, indicating that the localized states density of thin film is small and has good semiconductor performance; (4) as the Si doping concentration increases, the conductivity of the film improves. Based on the preparation of high-quality SnO2: Si thin films, the SnO2:Si/SnO2:10 at.% Ga homojunctions with different Si doping concentrations were prepared by the solution method in this paper. Research shows that: Si doping can optimize device performance, SnO2:15 at.% Si/SnO2:10 at.% Ga homojunction shows the best performance, the turn-on voltage is as low as 5.6 V, and it has good rectification characteristics. The photoresponse characteristics of SnO2:15 at.% Si/SnO2:10 at.% Ga homojunction under different frequency light was studied. The response to purple light (420 nm) is strong and the current increases by two orders of magnitude than the results In a dark environment, the response current is of the order of 10 −9 , and the response current of the homojunction excited by red, yellow, and purple light is significantly improved by a maximum of two orders of magnitude. With the increase of the frequency of light, the response current of the homojunction gradually increases. This may be due to the increase of the concentration of photo-generated carriers, which increases the probability of carriers moving to the space charge region to a certain extent and reduces the on-resistance of the homojunction. The excitation of the purple light increases the response current of the homojunction by two orders of magnitude, which is due to: (1) the SnO 2 : Si film has a significantly stronger absorption of purple light than red and yellow light; (2) the energy of purple light is higher than that of red and yellow light, resulting in a significant increase in photocarrier concentration. In addition, the turn-on voltage of the homojunction under the purple light excitation is significantly reduced to 5.0 V, and the homojunction shows more excellent rectification characteristics. The open circuit voltage (V OC ) is about 1.5 V and the short-circuit current (I SC ) is about 1.23 × 10 −10 A in this analysis. Compared with the UV detector based on p-NiO/n-ZnO structure prepared by Zhang et al. [30], which shows the performance that the photogeneration current increases to dark current about twice with ultraviolet light illumination and the V OC is about 0.43 V, the SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction in this research shows better photodetector performance. Besides, the SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction has weak light response to the red and yellow light bands, which is conductive to enhancing the reliability of the photodetector. The above experimental results show that the SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction has the potential to be used as a near-ultraviolet detector. tested in the dark environment, indicating that SnO 2 :15 at.% Si/SnO 2 :10 at.% Ga homojunction has the potential for applications in near-ultraviolet detectors.